A time-reversal (TR) communication system can include of multiple wireless devices that employ the TR principle to communicate with each other. One device, device A, the intended receiver of a data frame, first transmits a pre-defined channel probing signal (e.g., a pulse or a special code sequence) to another device, device B, the device that intends to send the data frame. As the channel probing signal propagates through the wireless channel, the channel probing signal is convolved with the channel's impulse response. Device B receives the channel probing signal and computes a location-specific signature waveform based on the received channel probing signal. In some implementations of a TR system, the location-specific signature waveform will be the time-reversed and conjugated version of the channel's impulse response, but other types of location-specific signature waveforms may also be computed. When sending the data frame to device A, device B convolves its data signal with the location-specific signature waveform and sends the resulting signal to device A. Device A then receives the transmitted signal and demodulates and decodes the data frame. Each of devices A and B can transmit and receive wireless signals. Each of devices A and B has transmitter circuit for transmitting signals and a receiver circuit for receiving signals. The transmitter circuit includes a transmitter antenna and a radio frequency chain for processing signals to be transmitted. The receiver circuit includes a receiver antenna and a radio frequency chain for processing received signals. The transmitter antenna and the receiver antenna can be the same antenna, or be different antennas. The transmitter circuit and the receiver circuit are referred to as “transceivers.”
If the above transmission principle is used, the wireless channel acts as a natural matched filter, creating an energy peak at device A's location. Since the transmitted signal was computed based on the wireless channel between device A and device B, this energy peak will only appear at the location of device A (and the nearby vicinity). At all other locations, only a small fraction of the transmitted energy will be received. This is the spatial focusing effect, in which constructive interference of the transmitted signal occurs only at the intended receiver's location, and the transmitted signal strength decreases significantly at other locations that are a few wavelengths away from the intended receiver's location.
The benefits of the spatial focusing effect are manifold. First, because the signal is concentrated at the intended location, the inter-user interference (IUI) can be significantly reduced, and simultaneous transmission to multiple users over the same frequency band becomes possible since users are naturally separated by their locations. Moreover, the energy concentration can also lead to better energy efficiency and enhanced physical layer security.
Even though the principles of TR communications were described using a unicast (point-to-point) transmission example between device A and device B, TR is not limited to unicast communications. It can be used for broadcast and multi-cast (point-to-multipoint) communication as well.
Massive multiple-input multiple-output (MMIMO) wireless communication systems can also achieve spatial concentration of the transmitted signal energy. The MMIMO systems employ a large number (several hundreds) of transmit and/or receive antennas and create high-dimensional quasi-orthogonal overall channel impulse responses for different receiver locations, focusing the energy of the transmitted signal at the desired receiver device's location. The benefits of MMIMO spatial focusing can be similar to those of the TR approach.
MAC design defines the transmission protocol over a shared medium and an efficient MAC protocol can optimally distribute the shared system resources to different users, coordinate their transmission, and provide QoS guarantee. Because of the unique spatial focusing effect, conventional MAC designs may no longer be efficient in MMIMO and TR systems.